Auto merge of #84004 - mattico:print-target-features-improvements, r=petrochenkov

Categorize and explain target features support

There are 3 different uses of the `-C target-feature` args passed to rustc:
1. All of the features are passed to LLVM, which uses them to configure code-generation. This is sort-of stabilized since 1.0 though LLVM does change/add/remove target features regularly.
2. Target features which are in [the compiler's allowlist](https://github.com/rust-lang/rust/blob/69e1d22ddbc67b25141a735a22a8895a678b32ca/compiler/rustc_codegen_ssa/src/target_features.rs#L12-L34) can be used in `cfg!(target_feature)` etc. These may have different names than in LLVM and are renamed before passing them to LLVM.
3. Target features which are in the allowlist and which are stabilized or feature-gate-enabled can be used in `#[target_feature]`.

It can be confusing that `rustc --print target-features` just prints out the LLVM features without separating out the rustc features or even mentioning that the dichotomy exists.

This improves the situation by separating out the rustc and LLVM target features and adding a brief explanation about the difference.

Abbreviated Example Output:
```
$ rustc --print target-features
Features supported by rustc for this target:
    adx                         - Support ADX instructions.
    aes                         - Enable AES instructions.
...
    xsaves                      - Support xsaves instructions.
    crt-static                  - Enables libraries with C Run-time Libraries(CRT) to be statically linked.

Code-generation features supported by LLVM for this target:
    16bit-mode                  - 16-bit mode (i8086).
    32bit-mode                  - 32-bit mode (80386).
...
    x87                         - Enable X87 float instructions.
    xop                         - Enable XOP instructions.

Use +feature to enable a feature, or -feature to disable it.
For example, rustc -C target-cpu=mycpu -C target-feature=+feature1,-feature2

Code-generation features cannot be used in cfg or #[target_feature],
and may be renamed or removed in a future version of LLVM or rustc.

```

Motivated by #83975.
CC https://github.com/rust-lang/rust/issues/49653

1 files changed, 15 insertions, 18 deletions

diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index b8d5b50dcb5..617b2ed970e 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -404,26 +404,21 @@ extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef TM) {
   printf("\n");
 }
 
-extern "C" void LLVMRustPrintTargetFeatures(LLVMTargetMachineRef TM) {
+extern "C" size_t LLVMRustGetTargetFeaturesCount(LLVMTargetMachineRef TM) {
   const TargetMachine *Target = unwrap(TM);
   const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
   const ArrayRef<SubtargetFeatureKV> FeatTable = MCInfo->getFeatureTable();
-  unsigned MaxFeatLen = getLongestEntryLength(FeatTable);
-
-  printf("Available features for this target:\n");
-  for (auto &Feature : FeatTable)
-    printf("    %-*s - %s.\n", MaxFeatLen, Feature.Key, Feature.Desc);
-  printf("\nRust-specific features:\n");
-  printf("    %-*s - %s.\n",
-    MaxFeatLen,
-    "crt-static",
-    "Enables libraries with C Run-time Libraries(CRT) to be statically linked"
-  );
-  printf("\n");
+  return FeatTable.size();
+}
 
-  printf("Use +feature to enable a feature, or -feature to disable it.\n"
-         "For example, rustc -C -target-cpu=mycpu -C "
-         "target-feature=+feature1,-feature2\n\n");
+extern "C" void LLVMRustGetTargetFeature(LLVMTargetMachineRef TM, size_t Index,
+                                         const char** Feature, const char** Desc) {
+  const TargetMachine *Target = unwrap(TM);
+  const MCSubtargetInfo *MCInfo = Target->getMCSubtargetInfo();
+  const ArrayRef<SubtargetFeatureKV> FeatTable = MCInfo->getFeatureTable();
+  const SubtargetFeatureKV Feat = FeatTable[Index];
+  *Feature = Feat.Key;
+  *Desc = Feat.Desc;
 }
 
 #else
@@ -432,9 +427,11 @@ extern "C" void LLVMRustPrintTargetCPUs(LLVMTargetMachineRef) {
   printf("Target CPU help is not supported by this LLVM version.\n\n");
 }
 
-extern "C" void LLVMRustPrintTargetFeatures(LLVMTargetMachineRef) {
-  printf("Target features help is not supported by this LLVM version.\n\n");
+extern "C" size_t LLVMRustGetTargetFeaturesCount(LLVMTargetMachineRef) {
+  return 0;
 }
+
+extern "C" void LLVMRustGetTargetFeature(LLVMTargetMachineRef, const char**, const char**) {}
 #endif
 
 extern "C" const char* LLVMRustGetHostCPUName(size_t *len) {